The invention is in the area of tertiary oil recovery techniques, in particular, an improved apparatus for downhole injection of steam into boreholes.
In the art of recovering oil from earth formations, tertiary methods are increasing in their importance. Initially, oil flow from many wells is driven by the pressure due to natural gases trapped along with the liquid oil in the formation. With the passage of time, natural gas pressures decrease. When gas pressure is insufficient to drive oil to the surface, pumping methods are then employed. As time passes, pumping methods may be ineffective because the flow of oil underground out of porous formations into a well may be very slow. It is at this point that tertiary methods are sought to accelerate the flow of oil from the formation into the well. A particularly useful tertiary method employs the injection of steam. Steam serves to heat the oil in the formation, thereby reducing its viscosity and increasing its flow rate into the well for recovery.
Methods employing downhole generation of steam within a well have proved to be particularly advantageous. The prior art discloses representative methods and apparatus.
In U.S. Pat. No. 3,456,721, Smith discloses a downhole burner for generating steam. Gaseous or liquid fuels are mixed with air and combusted in a burner with simultaneous spraying of water toward the flame. The water is sprayed from a cylindrical water jacket through a plurality of orifices. Steam is formed by the vaporization of the water as the water bombards the flame.
In U.S. Pat. No. 3,980,147, Gray discloses a downhole steam injector employing the combustion of hydrogen with oxygen to generate heat to vaporize injected water to form steam. The water moves in a single direction through an annular preheater jacket surrounding the combustion chamber, and, after being preheated, enters the combustion chamber through a plurality of grooves or passages at the top of the combustion chamber near the igniter and the hydrogen/oxygen flame.
Hamrick et al in their related U.S. Pat. Nos. 3,982,591 and 4,078,613 disclose downhole steam generators. In the first patent, in FIG. 17, water is injected through a plurality of apertures directly into a flame in a hydrogen/oxygen combustion zone. In the second patent, as shown in FIG. 2B, the oxidant is injected by an outer concentric tube surrounding the fuel nozzle. The envelope of oxidant around the steam of fuel tends to inhibit good mixing in the combustion zone. The water moves through a cooling annulus in a single direction before it is injected into a mixing zone spaced below the combustion zone. The mixing zone is defined by a cylindrical wall which has a plurality of apertures through which water from the cooling annulus passes laterally into the mixing zone. A heat-resistant liner is placed along the interior of the combustion zone.
Several problems have been encountered with these prior art downhole steam generators. One problem is the necessity of supplying air (the oxidant) downhole at relatively high pressure requiring expensive high pressure surface compressors. The air pressure required for the operation of the steam generator increases with increasing well depth. Providing greater downhole air pressure for deeper wells requires greater consumption of fuel to drive the high pressure compressors. Ecology problems are associated with atmospheric exhausting of fuels burned to operate the surface compressors.
A related problem, as pointed out above in relation to the Hamrick et al patent '613, is getting a good mixture of the fuel and oxidant for more complete combustion.
Another problem relates to overheating of the boundary layer adjacent the inner wall of the combustion zone. The boundary layer which is thick and of low velocity leads to deterioration of combustion chamber walls and excessive thermal conduction from the combustion zone to pre-combustion areas.
A problem prevalent with the prior art devices employing heat-resistant combustion zone liners is that the liners are not cooled adequately by adjacent heat transfer jackets through which water flows in a single direction. As a consequence, the liners cannot withstand the prolonged high temperatures from the combustion zone and undergo severe deterioration.
Problems are also encountered relative to the efficient preheating of the fuels and water used in the downhole steam generator. To explain, liquid fuels may be relatively cold at the surface prior to pumping downhole. As a result, the combustion process itself must give up heat to the liquid fuel to bring it up to combustion temperatures. Cool fuel results in production of soot, which is undesirable because of surface air pollution or clogging of pores in the earth formation. Similarly, water may be relatively cold at the surface prior to pumping downhole. As a result, a considerable portion of the heat generated by the combustion process is consumed in bringing the water up to the boiling point. Thus, less energy is available for driving high enthalpy steam into the earth formation.
Conditions downhole may occasionally occur which tend to flood the combustion chamber with reservoir fluids. This occurs particularly when a temporary interruption of combustion such as a flameout is experienced. A need for efficient means for isolating and protecting the combustion chamber is thus indicated.